To enable a pulverulent metal compound to form a high quality coating on a substrate, the powder must be uniformly distributed, it must effectively decompose at a temperature below about 650.degree.-700.degree. C. if the substrate to be coated is glass and further, the compound must contain a sufficient proportion of metal so as to quickly form an oxide layer of appreciable thickness, particularly if the substrate is moving rapidly in relation to the powder distribution means, as is the case for a glass ribbon at the outlet of a float bath.
Coatings of tin oxide on glass substrates, produced by the pyrolysis of dibutyltin oxide (DBTO) or dibutyltin fluoride (DBTF) powders are frequently employed with a view to reinforcing the surface of the glass, staining it and for providing such surfaces with specific optical and/or electrical characteristics, most notably, low emissivity. The use of powdered DBTO to produce such coatings is disclosed in French Patent Application Nos. 2,380,997 and 2,391,966 whereas the use of DBTF is disclosed in European Patent No. 39,256 and French Patent Application No. 2,542,636.
While both dibutyltin oxide and dibutyltin fluoride powders are capable of forming satisfactory layers, such layers develop colored reflections when they are deposited in the thickness necessary to obtain advantageous electronic properties. Further difficulties arise because slight variations in the thickness of the layers causes color irregularities and further, the color which is produced is often either not aesthetically pleasing or it may not be adapted to the style of the surrounding structure.
In order to obtain perfect color uniformity the reaction conditions under which the DBTO and DBTF powders are deposited must be scrupulously monitored. In particular, the powder must be of a consistent high quality and the spraying mechanism utilized to distribute the coating must be precisely adjusted so as to deposit a constant amount of powder upon the substrate.
In order to prevent the development of colored films on glass substrates and to avoid the necessity of implementing stringent monitoring techniques which are both expensive and time-consuming, applicants have developed a method wherein a powder containing indium formate may be distributed upon a hot substrate such as a glass ribbon passing the output of a float bath and which, upon the resultant pyrolysis of the powder, forms a thin, transparent, electroconductive coating upon the substrate.
Moreover, the pyrolysis of the indium formate compound may be performed with a sufficiently high degree of efficiency so as to be compatible with the rapid passing of the substrate past the point at which the coating is produced. This is in contrast to prior attempts at depositing pyrolyzed indium compounds upon glass substrates wherein solubilized indium acetylacetonates were found to pyrolyze at speeds insufficient for depositing a metallic oxide coating on a glass ribbon passing the output of a float bath at high speed.